The present invention relates to an optical filter such as an optical demultiplexing filter, an optical multiplexing filter, and an optical add/drop filter, and, in particular, relates to an optical filter with a function of suppressing dispersion.
FIG. 8 shows a schematic view of a prior-art optical demultiplexing filter. The optical demultiplexing filter shown in FIG. 8 comprises a chirped Bragg grating (CBG) 31 coupled to a three-port optical circulator 32. A wavelength division multiplexed (WDM) optical signal (wavelengths xcex1, . . . , xcexn) input to a first port P1 of the optical circulator 32 passes through a second port P2 to be input to a short wavelength end 31S (or a long wavelength end 31L) of the CBG 31. Only a wavelength component of the WDM optical signal having a wavelength xcexm (1xe2x89xa6mxe2x89xa6n) which corresponds to the grating pitch of the CBG 31 is reflected, while the other wavelength components of the WDM optical signal having wavelengths other than xcexm pass through the CBG 31 to be absorbed by a termination 33 and terminate optically. The reflected wavelength component (that is, the optical signal of the wavelength xcexm) is input to the second port P2 and is output via a third port P3 of the optical circulator 32. In this manner, the optical signal having the particular wavelength xcexm can be extracted from the WDM optical signal. The CBG 31 which has a chirped structure providing a gradually changing Bragg grating pitch is used here in order to secure a certain width for the pass band (or reflection band) of the optical signal.
However, when the optical demultiplexing filter constructed in a manner illustrated in FIG. 8 is used with a transmission system having a transmission rate which is as high as up to 40 Gbits/s, there is a problem that an impermissible amount of dispersion occurs in the output optical signal. FIGS. 9A and 9B show a cause of the occurrence of dispersion. Specifically, the dispersion is caused by the fact that the carrier frequency and the sidebands including upper and lower sidebands (USB and LSB) in the input optical signal as shown in FIG. 9A are reflected from different lengthwise positions in the CBG 31, as illustrated in FIG. 9B.
It is an object of the present invention to provide an optical filter that is capable of processing an optical signal while suppressing the occurrence of dispersion in the optical signal.
According to the present invention, an optical filter comprises a first chirped Bragg grating including a first input/output end and a second input/output end; a second CBG of the same structure as the first CBG, including a third input/output end of the same structure as the first input/output end of the first CBG and a fourth input/output end of the same structure as the second input/output end of the first CBG; and a circuit including an input port and an output port. The circuit receives a first optical signal via the input port and brings the first optical signal to the first input/output end of the first CBG. The circuit receives a second optical signal which is produced by reflecting at least one wavelength component of the first optical signal from the first CBG and brings the second optical signal to the fourth input/output end of the second CBG. The circuit receives a third optical signal which is produced by reflecting the second optical signal from the second CBG and outputs the third optical signal via the output port.
The optical filter may further comprise a termination coupled to the second input/output end of the first CBG, which does not reflect any optical signal; and a port coupled to the third input/output end of the second CBG, to which a fourth optical signal is input; wherein the circuit outputs a multiplexed optical signal including the third optical signal and the fourth optical signal via the output port.
According to another aspect of the present invention, an optical filter comprises a first CBG including a first input/output end and a second input/output end; a second CBG of the same structure as the first CBG, including a third input/output end of the same structure as the first input/output end of the first CBG and a fourth input/output end of the same structure as the second input/output end of the first CBG; a first circuit including a first input port and a first output port, wherein the first circuit receives a first optical signal via the first input port and brings the first optical signal to the first input/output end of the first CBG, the first circuit receives a second optical signal which is produced by reflecting at least one wavelength component of the first optical signal from the first CBG and brings the second optical signal to the fourth input/output end of the second CBG, and the first circuit receives a third optical signal which is produced by reflecting the second optical signal from the second CBG and outputs the third optical signal via the first output port; and a second circuit including a second input port and a second output port, wherein the second circuit receives a fourth optical signal via the second input port and brings the fourth optical signal to the third input/output end of the second CBG, the second circuit receives a fifth optical signal which is produced by reflecting the fourth optical signal from the second CBG and brings the fifth optical signal to the second input/output end of the first CBG, and the second circuit receives a sixth optical signal which is produced by reflecting the fifth optical signal from the first CBG and outputs the sixth optical signal via the second output port. The second circuit outputs the first optical signal, from which the second optical signal is removed and which has passed through the first CBG, via the second output port together with the sixth optical signal.